The accuracy with which a predetermined flow of particles is adjusted in terms of mass or volume is of considerable importance in various fields of process engineering, and the levels of requirements differ. When conveying solid particles through pipelines, for example, metering screws, rotary vane feeders or metering valves are used wherein, however, periodic fluctuations superposed on the outflowing flow of solid mass may occur and may prove disadvantageous. Also in other processes, such as in feeding fluidized, and optionally heated, solid particles into a reaction space, a controlled mass flow is essential. Another application is mixing where different components in different and precise amounts have to be added.
If fluid flows from bottom to top through a bed of solid particles, the particles will be entrained when its velocity reaches a certain level which is called the incipient fluidization velocity: the bed of particles is loosened and will be fluidized. From the fluidization point, the pressure drop above the fluidized-bed thus formed remains essentially constant and is dependent on the volume flow of the fluid.
The behavior of a gas/solid stream flowing out of a fluidized-bed as well as the type of possible flow states, their tendency to destabilization and measures for counteracting this phenomenon have been investigated both theoretically and empirically in various publications. In particular, standpipes which permit feeding of a solid stream via outflow orifices at their lower end--in particular with a variable cross-section--and which act as a pressure barrier against the reaction vessel, are used for passing fluidized solids from fluidized beds into reaction or treatment spaces which, in general, are under a certain positive pressure. Extensive investigations [see P. J. Jones, L. S. Leung "Fluidization", Cambridge University Press, 1978; "Fluidization", Academic Press, London, 1985; D. P. O'Dea, V. Rudolph, Y. O. Chong, "Powder Technology", 62, (1989) 291] have been made on the conditions under which the various flow states will occur within such standpipes. The coexistence of different forms of flow in a standpipe was detectable on the basis of X-ray photographs (M. R. Judd, P. D. Dixon, The American Inst. of Chem. Eng., 1978), that is, a loosely packed bed just above the constricted outflow orifice coexisting with a flow state in which solid particle strands flow downward at a high rate. The flow field, as such, is therefore by no means homogeneous, and the properties of the solid particle flow through the standpipe are no longer comparable with those of the fluidized particles in the fluidized-bed.
Other investigations: have been concerned with the question of the effect of outflow orifices or nozzles out of a fluidized-bed on the flow state of the gas/solid stream. Thus, when such an orifice is approached by the mass flow within the fluidized-bed, the porosity is supposed to decrease so that the flow state of the gas/solid stream before and after the orifice will differ. Furthermore, a certain proportion of gas will be found in the outflowing gas/solid stream (see R. J. Burkett, P. Chalmers-Dixon, P. J. Morris, D. L. Pyle, Chem. Eng. Sc., 1971, Vol. 26, 405). The mass flow of solid particles flowing through an orifice of a fluidized-bed apparatus will depend not only on the particle properties but also essentially on the height of the fluidized-bed above such orifice and its diameter, but is, up to a certain orifice's diameter, independent of the velocity of fluidizing gas. If a gas/solid stream does not flow through simple orifices of a fluidized-bed apparatus but thorugh nozzles, the proportion of gas in the gas/solid stream will depend on the ratio of the diameter of the nozzle to its length (see L. Massimilla, "Fluidization", Acad. Press, N.Y., 1971).
EP-A-0 084 887 describes a flow controller for solids which flow out of a fluidized-bed apparatus and through a standpipe into a solids consumer. Here a pressure medium is passed into a control hopper associated with a standpipe and having a plenum chamber; the resulting pressure difference between the control hopper and the solids consumer determines the through-put of solids. At the lower end of the standpipe or of the control hopper, a bed of solids is formed which cannot be compared with a homogeneous gas/solid stream out of a fluidized-bed.
Fluidized beds have been widely used in combustion, gasification and chemical gas/solid reaction applications of finely divided solids due to the optimally adjustable conditions and temperature stability. To be able to ensure controlled feeding of solid particles into treatment and reaction spaces, steady-state and homogeneous mixing are preconditions and, therefore, of decisive importance.
One field where exact metering is of importance is mixing of different materials where a precise proportion of the individual materials to be combined shall be maintained. This problem can be solved, in many instances, by means of volumetric dosage where units of volume of a predetermined size are fed into a mixing apparatus. However, volumetric dosage suffers under an inherent unprecision if the bulk densities of solids to be combined are rather different. To solve this problem, it would be conceivable to meter according to weight. This, however, has as a precondition that the material is present as a mass or bulk which in turn may render mixing more difficult, particularly of materials which tend to cluster or to adhere so as to form lumps when mixed with a liquid. This is, for example, the case with fly ashes. Certainly, the use of continuous operating balances would be conceivable, but it is known that precision of such type of balances is very limited.